An image capturing device (a digital image capturing device) designed to store captured images as digital data (image data) is provided with a function of compressively coding the image data to make a data volume smaller and then storing it, and thereby effectively using a data area in a storage medium. One of such compression coding methods is an intra-frame predictive coding system called JPEG (Joint Photographic Experts Group). This system is mainly used when a still picture is compressed, and there is a system called Motion JPEG in which this is applied to moving pictures. These coding systems are worse than MPEG (Moving Picture Experts Group) or others in compression efficiency at a same bit rate. Since these coding systems do not use information about differences between pictures, they have the advantages that processes at a time of coding and a time of decoding are light, and it is possible to easily edit an arbitrary portion. The present invention premises that an intra-frame coding system such as JPEG is used.
Some image capturing devices have a function of continuously capturing images by continuously pressing a release switch. In such continuous image capturing, a lot of image data is generated at one time, and thus a compression coding process needs to be properly controlled so that the data volume of the generated image data will be within a desired data volume. In the prior art, the following process is performed. Actions such as provisionally subjecting a plurality of pieces of image data (raw data) which occur after image capturing to a compression coding process with using a predetermined parameter about compression, and changing the parameter in response to a result thereof and implementing the compression coding process again are iterated so that all the data volumes of pieces of continuously captured image data will be closer to a desired data volume (for example, patent document 1).
There is also a known technique designed so that a compression coding process on an image signal is performed in response to each of the maximum value and the minimum value for an initial value in a search range for a compression rate, and a correlation between the compression rate and the data volume occurring after the compression coding process is calculated on the basis of the data volume obtained by the compression coding process with the maximum value and the data volume obtained by the compression coding process with the minimum value, and a candidate of the compression rate providing a desired data volume occurring after the compression coding process is estimated (for example, patent document 2).
In addition, there is a disclosed technique designed so that to implement a compression coding process in a short time, taking out image data and calculating a compression rate are continuously performed, and the next-time compression coding process on the image data is performed according to the compression rate calculated immediately before an image capturing operation is done (for example, patent document 3). Likewise, there is a disclosed technique where a desired value and the data volume of one piece of image data occurring after a compression coding process are compared, and a next-time compression rate is decided so that the data volume will be approximately equal to the desired value (for example, patent document 4).